(1) Field of the Invention
The present invention relates to an interleaf-containing, fiber-reinforced epoxy resin prepreg. More particularly, the present invention relates to a fiber-reinforced epoxy resin prepreg which contains a surface-treated polyimide film as an interleaf and is suitable for the production of a laminate composite material having a high interlaminar shear strength, a high flexural breaking strength and a large deflection quantity and having a high strength and a high toughness.
(2) Description of the Related Art
A fiber-reinforced epoxy resin composite material
has a high specific strength and a high specific elastic modulus and therefore, this composite material is widely used for the production of sport goods and construction materials for airplanes. Especially, an epoxy resin matrix composite material composed mainly of tetraglycidylaminodiphenyl methane (TGDDM)/diaminodiphenylsulfone (DDS) is vigorously used as carbon fiber-reinforced plastics (CFRP) for airplanes.
Since a composite material of this type is excellent in the hot-wet characteristic required for a construction material for an airplane, the composite material is widely used, but the composite material is generally poor in the toughness and is defective in that the impact resistance is insufficient.
Various improvements for eliminating this defect have been proposed.
For example, the surface treatment of the fiber or modification (toughening or the like) of an epoxy resin is carried out, but as is well-known, it is difficult to maintain a good balance between the hot-wet characteristic and the toughness.
Stitching of a prepreg laminate is conducted, but this method is not suitable for construction of a complicated large-size article and is insufficient in the practical utility.
An idea of a prepreg having an interleaf layer as disclosed in Japanese Patent Application Laid-Open Specification Nos. 63229/85 or 231738/85 is one new technique overcoming the above-mentioned defect. However, this technique is defective in that the hot-wet characteristic of the interleaf layer is insufficient and formation of a uniform thin resin layer by coating is difficult.
Furthermore, Japanese Patent Application Laid-Open Specification No. 231738/85 discloses an interleaf of a thermoplastic resin, and it is taught that a polyimide film is used as the interleaf. Since a thin polyimide film having a uniform thickness is excellent in the hot-wet characteristic, the film is suitable as the interleaf.
A prepreg having an interleaf as described above is ordinarily prepared by press-bonding the interleaf to a fiber-reinforced epoxy resin prepreg in the B stage (the state where polymerization is conducted to such an extent that a liquid thermosetting resin is rendered dry). However, if this interleaf is used, the bonding strength between the above-mentioned epoxy resin prepreg and the interleaf is insufficient. Accordingly, a composite material prepared from the prepreg having this interleaf or by laminating such prepregs is improved in the toughness but is not improved in mechanical strength characteristics such as tensile strength, flexural strength and interlaminar shear strength, and therefore, the composite material is not satisfactory.